1. Technical Field
The present invention relates generally to implants and to implant insertion instruments. More particularly, the present invention relates to implants and implant insertion instruments having impact absorption elements adapted to reduce the impact forces realized on an implant during the placement of the implant into a bony defect or deficit. In a preferred embodiment, the implant is a spinal implant which is placed into a receiving bed formed in an intervertebral space.
2. Background of Related Art
The use of implants for the repair of bony sites in the body is known to those skilled in the art. Implants are formed of a variety of different biologically compatible materials including metals (e.g., stainless steel, titanium, etc.), ceramics, polymers, human or animal bone, including cancellous or cortical bone, and composites. Unlike an implant constructed from metal, an implant constructed from cancellous bone can not be used just anywhere in the body due to its reduced mechanical strength. As such, implants constructed entirely from cancellous bone are generally used in areas subjected to reduced levels of mechanical stress. In contrast, implants constructed entirely from cortical bone have the mechanical strength suitable for use in any load-bearing region of the body. Accordingly, depending on the intended site for implantation, implants may be constructed of different materials tailored to the characteristics most desired at the site of implantation, e.g., mechanical strength, osteoinduction, etc.
For example, intervertebral implants for fusing together adjacent vertebrae of the spinal column are well known in the art. Such implants are formed in a variety of different shapes and sizes and are configured for insertion into receiving beds formed in the various regions of the spine. Intervertebral implants are formed of a variety of different biologically compatible materials including metals (e.g., stainless steel, titanium, etc.), ceramics, polymers, human or animal bone, including cancellous or cortical bone, and composites. Due to its reduced mechanical strength, an implant constructed from cancellous bone can not be used in all locations in the spinal column. As such, implants constructed entirely from cancellous bone are generally used in the cervical region of the spine. In contrast, implants constructed entirely from cortical bone have the mechanical strength suitable for use in any region of the spine. However, due to its osteoconductive properties, it is more desirable to use a spinal implant constructed from cancellous bone where possible, than a spinal implant constructed from cortical bone.
Intervertebral bone implants should stabilize the intervertebral space and become fused to adjacent vertebrae. Further, during the time it takes for fusion, i.e. biological fixation of the vertebrae, to be completed, the implant should have enough structural integrity to maintain the intervertebral space without substantial degradation or deformation of the implant. The implant must also provide spinal load support between the vertebrae.
When mineralized bone is used in grafts, it is primarily because of its inherent strength, i.e., its load bearing ability at the recipient site. While bone offers much improved incorporation, the inherent brittle nature of bone resulting from a high mineral content, particularly load-bearing cortical bone, severely limits its potential deformation. This has led to the development of surface demineralized bone grafts. Surface demineralization helps the graft to conform to the surgical site, and may also advantageously increase the rate of bone incorporation.
The process of demineralizing bone grafts is well known in the art. The successful application of such bone is predicated on sound knowledge of its biologic properties and its capacity to withstand the stresses to which it will be subjected. Demineralizing bone, using for example, a controlled acid treatment, increases the osteoinductive characteristics of the bone graft. One downside of the demineralization process is that the bone graft loses mechanical strength during the demineralization process. Demineralization of an implant can result in a reduction of its mechanical strength (e.g., in its compressive strength) depending on the configuration of the implant. Depending on the depth of the demineralization zone, this reduction in mechanical strength can range in degree from the negligible to the point where the implant is no longer suitable for its intended application.
In addition, some bone treatment processes, such as irradiation and lyophilization, can work against conservation of the mechanical strength of bone and can lessen the bone's weight bearing properties.
In some cases, instruments are utilized to aid in the positioning and placement of implants into a recess or defect present in bone. For example, the placement of a bone implant between adjacent vertebrae can be quite difficult. The implant is contoured to fit into the intervertebral space between the adjacent vertebrae after removal of the damaged intervertebral disk. During the insertion phase of the procedure, the implant typically will be held with a clamp, forceps, or some other such device in order to place the implant into the entry of the disc space. The implant is then usually engaged by a rod or punch, the end of which is tapped by a mallet, which drives the implant into the disc space. Implants that are impacted into place are subjected to high peak loads from the impacting operation. Even moderate hammering can generate loads of several thousand pounds, and can cause cracking or breaking of the implant. The effects of the hammering are further evident when a partially demineralized bone implant, having reduced mechanical strength, is used.
Instruments for positioning implants in a receiving bed, including but not limited to a bed formed between adjacent vertebrae, include instruments for gauging the size of a receiving bed, instruments for grasping an implant, and instruments for driving an implant into the receiving bed. A common deficiency in each of these instruments is that they treat all implants as if they are made of metal, thereby leading to the application of excessive insertion forces on bone implants. When these surgical instruments are used for insertion of implants constructed from bone, these instruments can cause the implant to weaken, crack, or even splinter during the insertion procedure, which seriously complicates the surgical procedure. These complications may lead to a total failure of the implant and/or a significant decrease in obtaining a solid bony arthrodesis. Moreover, an entirely new implant may be required if damage to the implant is excessive.
Accordingly, a need exists for a series of improved implant insertion instruments which are configured to facilitate ease of insertion of a bone implant into an implant receiving bed, and which decrease the impact load applied to the implant during insertion. Such instruments are especially needed in the area of spinal surgery, to facilitate the insertion of an implant into the intervertebral space and decrease the risk of damage to the implant.